1. Field of the Invention
The present invention relates to burst mode optical receivers, and more particularly to a bottom level detection device for a burst mode optical receiver which is capable of determining whether an input signal is present and independently generating a reset signal for initialization in an inter-packet period in accordance with the determination.
2. Description of the Related Art
In order to more rapidly transfer a large amount of information to subscribers, next-generation communication techniques require an FTTH (fiber to the home) system. In such conventional FTTH systems, an optical line is installed to each home. However, a significant shortcoming of the conventional FTTH systems is that it is very costly to replacing existing subscriber networks composed of copper wires. In view of this cost, a passive optical network (PON) is considered an alternative for implementing a cost-effective FTTH system.
FIG. 1 shows diagram of a passive optical network. The passive optical network includes an OLT (Optical Line Termination), which may be located in a central office, a 1×N-passive optical splitter, and ONUs (Optical Network Units), located in a subscriber' premises.
Generally, in an optical multi-connection network, such as the passive optical network shown in FIG. 1, each node transfers a packet or data to a different node using a predetermined time slot. Such optical multi-connection networks are different from existing point-to-point links in that received data or packets are different in amplitude and phase from one another due to optical losses occurring on different transfer paths. The data is typically called burst mode data. In other words, a plurality of subscribers utilize one optical line in a time division multiplex manner, but a receiver, i.e., the OLT, on the line recognizes that each subscriber sends data at a random time. The incoming data packets are not constant in amplitude due to differences among paths to respective subscribers.
A conventional burst mode optical receiver is known that receives burst mode data which is different in amplitude and phase on a packet basis and restores the received burst mode data such that their packets are the same in amplitude and phase. Such burst mode optical receivers remove a DC blocking capacitor used in an AC coupling scheme of a general receiver to prevent losses of burst mode data resulting from charging/discharging times of the capacitor. The burst mode optical receiver also functions to extract a detection threshold as a reference signal for data detection from each received burst mode packet. The burst mode optical receiver further functions to restore data by amplifying it symmetrically with respect to the extracted detection threshold.
FIG. 2 schematically shows the construction of a conventional burst mode optical receiver. The conventional burst mode optical receiver includes an optical detector 10, a preamplifier or transimpedance amplifier (TIA) 1, an automatic threshold controller (ATC) 2 and a limiting amplifier 3.
The optical detector 10 acts to convert an input optical signal into a current signal.
The TIA 1 functions to convert the current signal converted by the optical detector 10 into a voltage signal. A transimpedance, which is an input current-to-output voltage ratio, is determined by a feedback resistor Rf connected between an input terminal of the TIA 1 and an output terminal thereof.
In the burst mode optical receiver, the TIA 1 is used in a DC coupling manner. An input signal is amplified by the TIA 1 and then branches off into two parts. One part is input to the ATC 2, which then extracts a detection threshold of a received packet therefrom.
The other part is DC-coupled and input to the limiting amplifier 3. The detection threshold, which is automatically changed according to the amplitude of the corresponding packet, is input to Vref of the limiting amplifier 3. The limiting amplifier 3 functions to amplify signals of different amplitudes input thereto to restore them to signals having a constant amplitude.
However, the above-mentioned conventional burst mode optical receiver has a disadvantage in that a reset signal for initialization in an inter-packet period is input through the use of an external additional circuit. This makes it difficult to accurately control a reset timing. In addition, the use of the external additional circuit makes the receiver circuitry complicated and increases the size of the receiver parts.
Accordingly, there is a need in the art for an improved burst mode optical receiver.